Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1755878Ab3G2OK7 (ORCPT ); Mon, 29 Jul 2013 10:10:59 -0400 Received: from aserp1040.oracle.com ([141.146.126.69]:24299 "EHLO aserp1040.oracle.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1751195Ab3G2OK5 (ORCPT ); Mon, 29 Jul 2013 10:10:57 -0400 Date: Mon, 29 Jul 2013 10:10:36 -0400 From: Konrad Rzeszutek Wilk To: Ian Campbell Cc: Stefano Stabellini , xen-devel@lists.xensource.com, alex@alex.org.uk, dcrisan@flexiant.com, linux-kernel@vger.kernel.org Subject: Re: [PATCH v3 1/2] xen/balloon: set a mapping for ballooned out pages Message-ID: <20130729141036.GB5848@phenom.dumpdata.com> References: <1374600466-3229-1-git-send-email-stefano.stabellini@eu.citrix.com> <1374602409.6623.138.camel@hastur.hellion.org.uk> <20130723190521.GA7073@phenom.dumpdata.com> <20130724145838.GM2518@phenom.dumpdata.com> <1374723067.32286.34.camel@hastur.hellion.org.uk> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <1374723067.32286.34.camel@hastur.hellion.org.uk> User-Agent: Mutt/1.5.21 (2010-09-15) X-Source-IP: acsinet22.oracle.com [141.146.126.238] Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 3265 Lines: 74 On Thu, Jul 25, 2013 at 04:31:07AM +0100, Ian Campbell wrote: > On Wed, 2013-07-24 at 10:58 -0400, Konrad Rzeszutek Wilk wrote: > > On Wed, Jul 24, 2013 at 12:05:05PM +0100, Stefano Stabellini wrote: > > > On Tue, 23 Jul 2013, Konrad Rzeszutek Wilk wrote: > > > > On Tue, Jul 23, 2013 at 07:00:09PM +0100, Ian Campbell wrote: > > > > > On Tue, 2013-07-23 at 18:27 +0100, Stefano Stabellini wrote: > > > > > > +static int __cpuinit balloon_cpu_notify(struct notifier_block *self, > > > > > > + unsigned long action, void *hcpu) > > > > > > +{ > > > > > > + int cpu = (long)hcpu; > > > > > > + switch (action) { > > > > > > + case CPU_UP_PREPARE: > > > > > > + if (per_cpu(balloon_scratch_page, cpu) != NULL) > > > > > > + break; > > > > > > > > > > Thinking about this a bit more -- do we know what happens to the per-cpu > > > > > area for a CPU which is unplugged and then reintroduced? Is it preserved > > > > > or is it reset? > > > > > > > > > > If it is reset then this gets more complicated :-( We might be able to > > > > > use the core mm page reference count, so that when the last reference is > > > > > removed the page is automatically reclaimed. We can obviously take a > > > > > reference whenever we add a mapping of the trade page, but I'm not sure > > > > > we are always on the path which removes such mappings... Even then you > > > > > could waste pages for some potentially large amount of time each time > > > > > you replug a VCPU. > > > > > > > > > > Urg, I really hope the per-cpu area is preserved! > > > > > > > > It is. During bootup time you see this: > > > > > > > > [ 0.000000] smpboot: Allowing 128 CPUs, 96 hotplug CPU > > > > [ 0.000000] setup_percpu: NR_CPUS:512 nr_cpumask_bits:512 nr_cpu_ids:128 nr_node_ids:1 > > > > > > > > which means that all of the per_CPU are shrunk down to 128 (from > > > > CONFIG_NR_CPUS=512 was built with) and stays for the lifetime of the kernel. > > > > > > > > You might have to clear it when the vCPU comes back up though - otherwise you > > > > will have garbage. > > > > > > I don't see anything in the hotplug code that would modify the value of > > > the per_cpu area of offline cpus. > > > > You might have never onlined the CPUs and the kernel is built with DEBUG options > > which poison the page. > > > > Anyhow, doing a memset seems like a prudent thing to do? Perhaps when > > built with CONFG_DEBUG_XENFS you add poison values to it? > > The point is that the patches need for the per-cpu areas to *not* be > reinitialised over a vcpu unplug+plug, otherwise we will leak the > original page when we allocate the new one on plug. OK. > > We can't just free the page on vcpu unplug because it might still be in > use. I am still worried about before-the-cpu-is-up-the-per-cpu-has-garbage case. We could add code in the boot-before-smp (so when there is only one CPU) to do: for_each_possible(cpu) memset(__per_cpu(some_memory),0,sizeof...); and then I think it satisfies your concerns and mine? > > Ian. > -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/